I’m doing several scenes which take place in an outdoor shed with some waterwheel-driven machinery. Characters move here and there. There’s a bucolic scene in the background, a flowing stream… And hours of render-time. %|
I was able to drastically reduce that time by using compositing. It’s like the “blue screen” technique they use in movies. After building the original blend-file, I made copies of it so that I could generate, in separate .TGA file directories, these layers: - The background, which since it doesn’t move, is just one frame. - The moving stream, which animates in about 26 shots. - The machinery and wheel, which animate in 45 shots for one rotation, with motion-blur. - The people. Each is shot to a .TGA file (could also be .PNG; can’t be Jpeg or AVI-Jpeg) with the RGBA button turned on.
In each case, only the necessary layers are turned on. The rest is world-color, blue-screen. Significantly, the Alpha value for the action is 1.0; for the blue background it is zero.
In one case, where the machinery is sandwiched behind a foreground, in that shot (of the machinery) the materials of the foreground object are altered to have Alpha=0 but those layers are turned on. So the object itself forms a mask for the stuff that is behind it. Only that shot is rendered with motion-blur.
The pieces are composited together in the Sequence Editor, using the AlphaOver and/or AlphaUnder filters.
Notez vous: You must use a file-format that contains support for the “Alpha” information, such as .TGA or .PNG. And you must request that it be generated by pressing the RGBA button.
If you output to .TGA or .PNG these are separate files, one for each frame. Add them to Sequence Editor as Images, not Movie, and specify (say) “*.tga” as the filename in the requestor to select all the files. Blender will understand…
I’m still tweaking the movement of our people in this shot, but when I do so the only thing I have to re-render is the people.
By: - rendering only what I need (one frame of background, 26 frames, 45 and so-on), and repeating those strips as needed; - generating only the machinery-in-motion layer with motion-blur; - avoiding repetitive re-rendering of things that don’t change… I have been able to slash the render-time for this and other shots, and become much more productive.
I can see applications for this technique even in static shots. The specular “gleam” on a surface could be rendered using a particle-system … in a layer with nothing else. Hard-to-render foreground objects could be rendered by themselves, and sandwiched against simpler backgrounds. You can render “Only Shadows” and paste the shadows or the highlights (“Just the shadows, ma’am”) onto an object that was otherwise rendered using cheap, fast, shadow-free area lights.
Any technique that delivers good results in less time is just fine with me, and this certainly appears to be one.